Frank Jackson first presented
the Knowledge Argument (henceforth KA) in "Epiphenomenal Qualia"
(1982). The KA is an argument against physicalism, the
doctrine that (very roughly put) everything is physical. The general
thrust of the KA is that physicalism errs by misconstruing or denying
the existence of the subjective features of experience. Physicalists
have given numerous responses, and the debate continues about whether
the KA ultimately succeeds in refuting any or all forms of physicalism.
Jackson himself has recently (1998a) recanted: he
now rejects the KA and endorses physicalism. One point should be
acknowledged by all sides: in formulating the KA, Jackson clearly
and forcefully articulated a deep-seated, intuitive reason why even
some scientifically-minded analytic philosophers have resisted physicalism.
The KA has been sufficiently influential that it is even discussed
in writings aimed at general audiences that include non-philosophers;
for example, it is discussed in E. O. Wilson's Consilience (1998).

What follows is an overview
of the literature on the KA. I will begin in section 1 with a sketch
of the KA, followed by a discussion of the historical background
in section 2. In section 3, the third and longest section, I will
provide a taxonomy of objections to the KA, along with brief descriptions
of them. In section 4 I will compare the KA to related arguments.
I will close in section 5 by briefly raising a question about the
extent to which the KA can be generalized.

1. The Basic Idea of the KA

The basic idea of the KA can
be put abstractly as follows: one might know all the objective,
physical facts about human conscious experiences, and yet fail to
know certain facts about what human conscious experiences are like
subjectively; therefore, there are facts about human conscious experiences
that are left out of the physicalist's story, and so physicalism
is false.

The KA's persuasive force
derives chiefly from Jackson's clever thought experiment involving
Mary, the super-scientist. (The Mary case is one of two of Jackson's
original cases. The other involved Fred, who could see more colors
than normal humans can. The Mary case is simpler and thus more often
discussed.) Mary spends her life in a black-and-white room and has
no color sensations. She watches science lectures on black-and-white
television and learns everything about seeing in color that can
in that way be learned. This includes mastering the completed science
of human color vision. If physicalism were true, she would know
all the facts about color experiences, because physicalism entails
that all such facts can be expressed in the colorless language of
science. But, one thinks intuitively, when she ventures into the
colorful outside world and has color experiences for the first time,
she learns something: she learns what it's like to see in color.
Therefore, Jackson concludes, ph ysicalism is false.

In short:

1. Before Mary leaves the room, she
knows all the physical facts about color experiences.

2.
When Mary leaves the room, she learns new facts (i.e., facts
she did not know previously) about color experiences --
facts about what it's like to see in color.

3. Therefore, there are non-physical
facts about color experiences.

4. Therefore, physicalism is false.

The argument can be formulated using the term
'information' instead of 'facts'; Jackson uses both locutions.

The preceding compressed summary
of the KA is convenient for conveying the basic idea, and it is
accurate insofar as it goes. But it hides some implicit assumptions,
as will be made clear below.

Before proceeding, however,
it should be noted that the Mary case involves at least some idealization
and possibly oversimplification. First, the assumption that science
is completeable, even in a limited realm like the science of color
vision, is not trivial. Second, several special implicit stipulations
must be made in order to ensure that Mary's pre-release visual experiences
are not in color. For example, we must assume that she never presses
on her eyes in such a way as to produce flashes of yellow. Alternatively,
we could dispense with the device of the black-and-white room, and
assume instead that she is congenitally colorblind and, after completing
her science lessons, acquires color vision. Third, it may be naive
to assume that Mary's visual experiences would be very much like
watching black-and-white television; for (anecdotal) reasons against
that assumption, see Sacks 1995. However, none
of these three points should be mistaken for subs tantive objections
to the KA. There are, of course, substantive objections, as we shall
see in section 3 below.

2. Historical Background

Something close to the KA
can be found in writings that preceded Jackson 1982. As Jackson himself
acknowledges, "Epiphenomenal Qualia" owes a great deal
to Nagel 1974. Indeed, David Lewis (1983) describes Jackson's
KA as a purified version of an argument in Nagel 1974, and there is much
truth in Lewis' description. Thus, authors sometimes employ phrases
like 'the Nagel-Jackson Knowledge Argument', and some (e.g., Pereboom
1994) argue that Nagel's and Jackson's arguments are at
root identical. The KA did not elicit an enormous response until
Jackson 1986, in which Jackson presented the KA for a second time
and defended it against objections raised in Churchland 1985.

For the reader unacquainted
with the history of Twentieth Century analytic philosophy of mind,
it is worth noting that the dominant theories have all been physicalist
-- or in the case of functionalism, compatible with physicalism
-- and they have usually taken a reductionist form (see Searle 1992). That is the principal
reason why arguments like the KA are regarded as important: the
KA is an intuitively forceful attack on the entire reductionist-physicalist
approach, rather than on one particular form of reductionist-physicalism,
such as philosophical behaviorism.

3. Objections to the KA

3.1. Outline of Objections

Objections to the KA have
been many and varied, and I will describe them below, beginning
with section 3.2. In this section, I will classify the objections
by explaining how their proponents would respond to a series of
questions about the KA (a similar taxonomy appears in van Gulick
1993). My basis for choosing these questions in particular
is as follows. The KA is driven by the intuition that Mary learns
something, i.e. that she acquires knowledge, when she leaves the
room. Objections may thus be divided into two groups: (i) those
that reject Jackson's intuition that Mary gains knowledge when she
leaves the room, and (ii) those that accept Jackson's intuition,
but reject the consequences that he infers from it. Group (i) is
represented by a negative answer to Question 1 below, and group
(ii) is represented by negative answers to Questions 2, 3, or 4.
(Authors are sometimes listed more than once, either because they
propose explicitly diff erent views or because their views can be
understood in different ways.)

#Question 1: When Mary
is released, does she acquire knowledge (in any sense)?

No. We think so only because we
fail to appreciate how much the pre-release Mary knows. (Dennett,
Churchland, Foss, Jackson?)

#Question 2: But does
she acquire factual (propositional) knowledge?

No. She gains only know-how, which
is not propositional. (Nemirow, Lewis, Mellor)

No. All the facts
about qualia are, though inaccessible to the pre-release Mary,
facts about the brain, and the existence of such facts is consistent
with non-reductionist forms of physicalism. (Searle, Flanagan,
Alter)

In the preceding chart, authors
who accept the KA's anti-physicalist conclusion are not listed.
Such philosophers include Robinson (1996), Chalmers (1996), Gertler (1999) and (until recently;
see section 3.2 below) Jackson (1982, 1986). Chalmers' discussion
is arguably the most thorough and vigorous defense of the KA as
a refutation of physicalism.

Let us now consider each question in turn.

3.2. Does Mary Acquire Knowledge When Released?

Most of those discussing the
KA are willing to grant that Mary learns something when released. But not everyone accepts that premise.
Foss (1989) argues that the pre-release Mary lacks no knowledge
about color experiences, because she could know everything that
the color-sighted people who reside in the colorful outside world
would (or even might) say about colors. Foss's strategy has not
been popular, presumably because, as Chalmers (1996) notes, it is far
from clear that knowing everything about the verbal behavior of
those who have had color experiences is sufficient for knowing what
it's like to see in color.

Other reasons for doubting
that Mary learns anything when she is released may be found in Churchland
1985, Stemmer (1989) and in Dennett
1991. The position is somewhat clearer in Dennett 1991, since Churchland
concentrates on another objection to the KA (see section 3.5 below).
Dennett argues that, prior to leaving the room, Mary is already
capable of identifying the kinds of color experiences she is about
to have by using technical instruments like cerebroscopes -- she
would be able to recognize the brain patterns stimulated by her
first color experiences, and so she would hence not be fooled by
a blue banana.

One problem with Dennett's
argument is that it seems to presuppose that having certain recognitional
capacities is equivalent to knowing what it's like to see in color.
That presupposition clearly requires defense and would beg the question
against Jackson if not defended on independent grounds; see below,
section 3.3. For other criticisms of Dennett's discussion of the
KA, see Robinson 1993 and Jacquette 1995.

Nevertheless, there is an
important moral to be drawn from Dennett's discussion: we should
take seriously the possibility that our intuitive judgment -- that
Mary learns something when released -- is based on ignorance of
what Mary's vast knowledge would involve. After all, her pre-release
knowledge includes everything in completed physics and neurobiology. Our confidence in any conclusions
we draw from the Mary case should be limited accordingly.

Jackson himself has recently
rejected the premise that Mary learns anything when she leaves the
room, partly based on similar considerations. In Jackson 1998b) he argues that
we should be suspicious of giving "intuitions about possibilities
[like the Mary case] too big a place in determining what the world
is like" (43-4). And in Jackson 1998a he states that,
in his view, Mary does not gain knowledge when she leaves the room.
He thinks that the real puzzle is to explain why the intuition to
the contrary is so strong. He suggests the following explanation.
Learning a physical fact often involves making inferences; it is
often a long and complex process. By contrast, when Mary leaves
the room, her gain in knowledge is almost immediate. We therefore
infer, wrongly but naturally, that the knowledge gained cannot be
knowledge of physical facts.

3.3. Does Mary Gain Only Abilities When
Released?

If it is granted that Mary
gains knowledge when released, the question arises as to what kind
of knowledge she gains. It is generally agreed that if she gains
knowledge, the knowledge she gains is knowledge of what it's like
to see in color, in Nagel's (1974) sense of the phrase.
(Nida-Rumelin (1998) argues that the
phrase 'knowing what it's like' should not be used in formulating
the KA, but I will ignore this complication.) But what kind of knowledge
is knowing what it's like?

Jackson assumes that knowing
what it's like is a kind of propositional knowledge, but others
disagree. In his review of Thomas Nagel's Mortal
Questions, Laurence Nemirow proposed
that knowing what it's like is a kind of know-how -- it consists
only in the possession of abilities, such as the ability to identify
red objects as red, to imagine or remember having a red experience,
and so on. Nemirow (1980, (1990) and Lewis (1983, 1988) adopt the ability
analysis of knowing what it's like and use it to block the KA. Mellor
(1993) also defends a version of the view. These philosophers
argue as follows. When Mary is released, she learns what it's like
to see in color, just as Jackson says. But what this means is that
she acquires new abilities, not new information: she learns no info rmation or facts that she did
not previously know. Therefore, they conclude, the Mary case provides
no basis for doubting physicalism's truth, even though the pre-release
Mary does not know what it's like to see in color.

Challenges to the ability
analysis of knowing what it's like are found in Conee 1994, Alter 1998, Loar 1990, Raymont 1999, and Lycan 1995 and 1996. Conee and Alter
argue that having the abilities Nemirow and Lewis mention is neither
necessary nor sufficient for knowing what it's like. For example,
against sufficiency Conee notes that one can have an ability without
ever exercising it, and such could be Mary's pre-release state.
Both Conee and Alter argue that in principle one could know what
it's like to see red while seeing a red tomato and never possess
the ability to imagine, remember, etc., such experiences; and that
therefore possessing such abilities is not strictly necessary for
knowing what it's like. Raymont 1999 offers similar
arguments and defends them against objections. The general strategy
here is to argue that knowing what it's like cannot be identified
with having abilities because there are conceivable cases in which
one can know what it's like without having the relevant abilities
and vice versa.

Lycan offers a barrage of
criticisms of the ability analysis -- ten in all, some of which
involve close semantic analyses of the relevant linguistic expressions.
One of these semantically-based criticisms was originally given
by Loar, who in turn modeled his objection on one of Geach's (1960) objections to
ethical emotivism. Loar and Lycan argue that ordinary English claims
expressing knowledge of what it's like can be embedded in conditionals
in a straightforward way, and that the same cannot be said of ordinary
English expressions of the possession of the relevant abilities.

One aspect of the Lewis-Nemirow
strategy that has not been much discussed is the assumption that
know-how consists entirely in the possession of abilities, as opposed
to propositional knowledge. That assumption has been forcefully
challenged by Noam Chomsky (though not in connection to the KA).
See, for example, Chomsky 1994. There Chomsky
discusses brain injuries that result in a temporary loss of an ability,
such as the ability to ride a bicycle, which is regained after recovery.
As Chomsky writes, "[w]hat remained intact was the cognitive
system that constitutes knowing how to ride a bicycle; this is not
simply a matter of ability, disposition, habit, or skill" (Chomsky
1994, 11). Chomsky is concerned principally with linguistic know-how,
of course, but (as his bicycle example indicates) his arguments
are general and therefore apply to the Lewis-Nemirow ability analysis
of knowing what it's like: if there is more to know-how than possessing
abilities, then one could question whether the Lewis-Nemirow strategy
succeeds in preserving the intuition that Mary gains knowledge in
any sense, even if it were granted that knowing what it's like is
a kind of know-how. This criticism is pursued in Alter n.d.

3.4. Does Mary Gain Only Acquaintance Knowledge
or Indexical Knowledge When Released?

A variation of the Lewis-Nerimow
strategy is to argue that, upon her release, Mary gains neither
propositional knowledge nor know-how, but rather acquaintance knowledge
-- that she comes to know color experiences in the sense that one
comes to know a person or a city (Conee 1994). Herbert Feigl
(1967, esp. p. 68) once proposed such an account of knowing
what it's like, and other authors have made similar proposals. Conee
(1994) applies the acquaintance analysis specifically to
the KA.

Conee's view is criticized
in Alter 1998, where it is argued that, although Mary may acquire
acquaintance knowledge upon her release, it is implausible that
all she
gains is acquaintance knowledge, and that this conclusion is supported
by a careful examination of Conee's analogy between becoming acquainted
with a person or city and becoming acquainted with color qualia.
However, the acquaintance knowledge analysis of knowing what it's
like cannot be easily dismissed; it should be regarded as a contender
view.

Some perceive a connection
between Mary's situation and a lack of indexical knowledge, and
that connection forms the basis of an objection similar to the one
based on the acquaintance knowledge analysis. These objectors (McMullen
1985, Bigelow and Pargetter 1990, Papineau 1993, Yi n.d.) concede that Mary
gains knowledge when she leaves the room, but they argue that her
gain is comparable to, and no more puzzling than, the absent-minded
U.S. historian who learns that today is July 4th, America's Independence Day. This strategy
is criticized by Chalmers (1996).

One difference between the
indexical knowledge strategy and Conee's acquaintance knowledge
strategy is that advocates of the former tend not to deny that knowing
what it's like consists (at least in part) in propositional knowledge.
Their tendency is to argue that the comparison of the Mary case
to other examples of gaining indexical knowledge shows that Mary's
apparent gain in factual knowledge does not indicate that color
experiences are non-physical. See section 3.7 below for similar
criticisms of the alleged anti-physicalist implications of the Mary
case.

3.5. Does Mary Just Come To Know Old Facts
Under New Guises?

Several of the KA's critics
admit that the knowledge Mary gains when released is propositional
in kind, but deny that she learns any new facts -- facts that were not known to her prior to
her release. According to these critics, what happens is that Mary
comes to represent
differently facts she already knew. On their view, the facts about
color experiences are captured completely and accurately by the
completed science that the pre-release Mary learns. Those same facts
can be represented under phenomenal guises, but the pre-release
Mary does not so represent those facts. The pre-release Mary lacks
knowledge about color experiences in something like the way that
Jones, who is up on his sports history but has never heard the name
'Cassius Clay', lacks knowledge of Clay's boxing talents. Jones
does not lack any pugilistic knowledge; he simply fails to represent
the relevant facts using the 'Clay'-guise. Likewise, the objectors
argue, the pre-release Mary knows all the fa cts about color experiences;
she simply fails to represent them under the relevant phenomenal
guises.

The old-fact/new-guise analysis
was first used as a criticism of the KA by Terence Horgan (Horgan
1984). Versions of it have since been developed by several
authors, including Churchland (1985), Pereboom (1994), Tye (1986), Bigelow and Pargetter
(1990), van Gulick (1993), Lycan (1990, 1996), Loar (1990), McMullen (1985), Papineau (1993), and Teller (1992).

It is argued in Alter 1998 and Chalmers 1996 that the analogies
drawn to cases like the Ali/Clay case do not support the old-fact/new-guise
theory. Alter and Chalmers each argue that, even if Mary gains "only"
new phenomenal guises when she leaves the room, she nevertheless
learns new facts involving those new guises. That criticism is not
sufficient to undermine Loar's sophisticated version of the old-fact/new-guise
analysis, as Chalmers points out; but Chalmers also argues that
even Loar's version of that the theory does not stand up to further
scrutiny. A criticism of Tye's version of the theory is presented
in Raymont 1995; a criticism of Pereboom's version is presented in
Alter 1995a; and at the 1999 Pacific Division
APA meetings, A. Anchustegui argued that the theory succumbs to
Kripke's modal argument against type-identity theory. However, the
old-fact/new-guise theory of Mary's post-release knowledge remains
the most widely held view among the KA's critics.

3.6. A Semantic Objection to the KA

Some adduce considerations
from the philosophy of language against the premise of the KA that
Mary learns new facts when she leaves the room. Those objectors
reason as follows. There is no reason why the pre-release Mary cannot
communicate with color-sighted people who reside in the colorful
world outside her black-and-white room. Those color-sighted people
can express in language precisely the facts about knowing what color
experiences are like that Mary is supposed not to know. For example,
they might say or write, "Seeing red is like this", intending the demonstrative to refer to color
qualia. Indeed, such a sentence might appear in one of Mary's science
lectures. According to the objectors, some such communication would
provide Mary with access to any facts about color experiences that
she does not learn from her science lectures. After all, the objectors
reason, contemporary theories of reference suggest that historical
chains of communication enable those who know vi rtually nothing
at all about Cicero to refer specifically to him (see, for example,
Kripke 1972); why, then, wouldn't Mary's communication with those
who have had color experiences provide her with cognitive access
to the facts in question?

Versions of this semantically-based
objection to the KA are presented in Tye 1986 and Conee 1994. Alter (1998) counters, however,
by arguing that the objection depends on confusing different senses
of 'having access to a fact' and on related mistakes. But the objection
raises important issues about the extent to which language can enable
one to grasp propositions about that with which one is unacquainted
-- a topic of particular interest to Bertrand Russell and other
central figures in philosophical semantics. See Russell 1910-11. For more recent
discussions of the issue, see Donnellan 1979 and Kaplan 1989.

3.7. Is Physicalism Thus Refuted?

The KA is sometimes portrayed
as an argument for something akin to Cartesian Dualism. Whether
or not the KA could be used for such a purpose, Jackson makes clear
that he never had any such intention. In Jackson 1986, he suggests that
the KA may be used to support property dualism, and David Chalmers
(1996) concurs (see also Furash 1989 and Robinson 1996). Unfortunately,
Jackson does not explain exactly what he means by 'property dualism'.
Minimally, property dualism implies that (certain) mental properties
are not identical
to any neural properties. But that non-identity thesis is consistent
with the weaker physicalist thesis that mental states are constituted by or
realized in
brain states; and the non-identity thesis is also consistent with
the thesis that disembodied minds are impossible.

If Mary learns new facts when
released, does it follow that physicalism is false? That depends
on what physicalism entails. Jackson (1986) defines physicalism
as the doctrine that all facts are physical (in Jackson 1982 he formulated the
physicalist thesis with '(correct) information' instead of 'facts',
but he treats the two formulations as equivalent). And he claims
that if physicalism is true, then all the facts about color experiences
would be known to the pre-release Mary -- a claim that may seem
trivial, but is not. What may be trivial, because it is stipulated
and seems coherent, is the claim that the pre-release Mary knows
everything that can be conveyed to and understood by a human being
by black-and-white television lectures. But why should we believe
that all physical facts can be conveyed to a human being -- or any
creature -- through a black-and-white medium?

That question has not received
a tremendous amount of attention, perhaps because Jackson formulates
his stipulation by saying that Mary learns all of the physical facts
while in the room. Yet one may legitimately wonder whether the latter
stipulation is coherent -- whether anyone, even a superscientist,
could learn all of the physical facts about color experiences without
having any color experiences herself. All Jackson does to defend
the coherence of his stipulation is to offer the following quick
reductio ad absurdum argument: if it were impossible to learn all of the
physical facts about color experiences without having any color
experiences, then the Open University would have to be broadcast in color, which is absurd. (The Open
University is a British University in which classes are conducted
almost entirely over television.) But Jackson's reductio argument is not compelling. As odd as it may sound,
perhaps some physical facts about color experiences cannot be conveyed
accurately and completely in black-and-white -- or perhaps some
such facts cannot be understood if conveyed in black-and-white. Perhaps the Open
University would have to be broadcast in color, if the goal is to
convey all the facts about color vision. Owen Flanagan (1992) makes this point,
arguing that the pre-release Mary "does not have complete physical
knowledge" (100). Similar points are made in Alter 1998, Horgan 1984 and Searle 1992. Bealer (1994) also uses similar
reasoning, along with a comparison of the KA to the paradox of analysis,
to conclude that the KA poses no threat to the mental-state/brain-state
identity thesis.

In a short postscript to Jackson
1986, Jackson (1995) elaborates on
his view that the epistemological premises of the Knowledge argument
support substantial metaphysical conclusions. More specifically,
he argues that, "materialism is committed to the a priori deducibility
of our psychological nature from our and our environment's physical
nature" (189). He elaborates on this point in Jackson 1998b. And some authors
do take the KA to refute, or at least provide a serious challenge
to, physicalism of any kind. But the question of what exactly follows
from admitting that Mary learns new facts when released remains
unresolved.

In Jackson's original article,
facts about functional roles were counted among the physical facts,
and the implication seems to be that Jackson did regard the KA as
refuting functionalism (for concurring opinions, see Vidal 1995 and Robinson 1993). Jackson (1982) could reasonably
be read as implying that the KA leaves us with epiphenomenalism;
in that article, immediately after presenting the KA, he defends
epiphenomenalism against objections. But whether the KA implies
epiphenomenalism is a substantive issue; see Searle 1992. Indeed, Watkins
(1989) argues that Jackson cannot consistently accept epiphenomenalism
and irreducibly non-physical qualia. More generally, there is no
consensus about what, if any, substantial metaphysical theses follow
from granting that Mary learns new facts when she is released.

4. The KA, Nagel's Argument, and Kripke's
Modal Arguments

I noted in section 2 that
the KA bears much similarity to arguments presented in Nagel 1974. Some differences
between the KA and Nagel's arguments are worth mentioning. First,
Nagel's argument involves claims about the essence of mental and physical processes. The KA involves
no such claims. In fact, in a review of a book by Brian O'Shaughnessy,
Jackson (1982b) suggests that although mental states have qualia,
qualia may be inessential properties of those states -- a view
that directly contradicts Nagel's opinion on this matter. Second,
unlike Nagel's arguments, the KA does not involve empirical theses
about what humans can and cannot imagine (Jackson emphasizes this
point in both Jackson 1982 and Jackson 1986). Third, unlike
Jackson, Nagel does not purport to show that physicalism is false;
Nagel's conclusion is rat he r that physicalism is, though possibly
true, presently unintelligible. This last point may, however, be
a distinction without a difference. Nagel can plausibly be read
as arguing for the falsity of reductionist forms of physicalism
that deny the subjectivity of the phenomenological features of mental
states, even though he regards non-reductionist forms (such as dual
aspect theory) as possibly true though presently unintelligible.
The KA could perhaps also be seen as directed only at such reductionist
versions of physicalism.

It is also important to distinguish
the KA from Kripke's (1972) famous anti-physicalist
arguments. As Jackson (1982) notes, the KA
is not a modal argument in the sense that Kripke's arguments are.
Unlike Kripke's arguments, the KA could consistently be given by
Quinean skeptics about de re modality or by contingent identity theorists.

However, John Searle (1992) claims that Jackson's
KA, Nagel's bat-arguments, Kripke's modal arguments, and certain
arguments of his (Searle's) own can be seen as variations on a single
theme: Twentieth Century materialist theories, from behaviorism
to the identity thesis to functionalism to eliminative materialism,
all err in denying the irreducible subjectivity of mental states
(see Holman 1987). Put more positively, the KA could be seen as one
variation of an argument for irreducible qualia. As Searle emphasizes,
that conclusion is at least prima facie consistent with the claim that qualia are features
of the brain.

5. Generalizing the KA

Jackson writes that the KA
can be deployed, "for the various mental states which are said
to have (as it is variously put) raw feels, phenomenal features
or qualia" (1982, 130). Surprisingly,
the question of exactly how far the KA extends has not been seriously
investigated. Virtually all of the published discussions of the
KA follow Jackson's in focussing exclusively on perceptual experiences
and sensations. (Janet Levin's (1985) paper on the KA,
"Could Love Be Like A Heatwave?", is no exception: love
is mentioned nowhere in the text of her article.) But there is a
substantive issue about whether Jackson's reasoning, to the extent
that it is sound, can be extended to other aspects of consciousness,
such as emotions and propositional attitudes. A brief discussion
of that issue occurs in Alter 1995b, and a more detailed
account appears in an as yet unpublished paper by Alt er, "What
a Vulcan Couldn't Know".

A tempting conclusion to draw
from the foregoing overview is that the issues surrounding the KA
have been thoroughly canvassed. But who knows? As Yogi Berra allegedly
said (according to Pinker 1997), it is hard to
make predictions, especially about the future.

[I would like to thank James
Otteson, Russell Daw, and Marco Nani for helpful suggestions. -T.A.]